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Why Choose a Soft Starter Instead of an AC Drive?

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In a previous blog, we examined why AC drives continue to be applied at a rate of approximately 20:1 even in situations when soft starters are sufficient to do the job.

The reason: AC drives will work in almost all applications, but soft starters will not. This does not mean that AC drives are easier to specify, install and commission - it simply means that AC drives work in a wider variety of applications.

So why choose a soft starter? Substantial savings can be realized when using a soft starter instead of an AC drive in the proper applications. These savings can come in:

  • Cost
  • Physical size / volume
  • Energy and efficiency

In order to reap the benefits of these savings, application characteristics are very important when choosing a soft starter or an AC drive. Items such as torque requirements, speed control and power distribution concerns must be carefully considered.

For soft starter applications, some general rules of thumb include:

  • Lower starting torque applications
  • Lightly or moderately loaded applications
  • Full speed operation
  • Reduction of mechanical wear and damage to system
  • Limiting current is prime reason for starting method

Significant cost savings can be realized when using a soft starter instead of an AC drive. A soft starter has silicon-controlled rectifiers (SCRs) that connect the AC input line to the motor. In contrast, AC drives have many more components – typically a diode front end, DC bus and insulated-gate bipolar transistors (IGBTs). More components simply means more cost.

Typical cost ratios of AC drives versus soft starters are given below (cost AC drive / cost soft starter)

  • Below 5Hp (3.7kW) 1.5 to 2.0
  • 5Hp (3.7kW) to 50 Hp (37kW) 2.0 to 4.0
  • 50Hp (37kW) to 250 Hp (185kW) 1.5 to 4.0
  • 250 Hp (185 kW) to 500 Hp (375kW) 3.0 to 4.5
  • Above 500 Hp (375kw) 2.5 to 7.0

Soft starters can also provide substantial physical size and volume savings over an AC drive. The additional components in an AC drive cost more, and they physically take up more space. Similar to the savings in cost, the savings in volume that a soft starter provides depends on the power rating:

  • Below 37 Amps: 45mm soft starter width is significantly narrower than AC drives of the same current
  • 100Hp (75kW): Most AC drives occupy 400% more space than typical soft starters
  • 30Hp (22kW), 500Hp (375kW), 800Hp (560kW) and 1000Hp (710kW) are most significant savings: Volume savings reach 500% to 1000%

Energy savings can also be realized by using a soft starter instead of an AC drive. Soft starters can achieve up to 99.5% to 99.9% efficiency.

Efficiency depends on the size of the soft starter and the 3-phase voltage applied. After the starting process is complete, a soft starter with an integrated bypass pulls in an internal contactor. The SCRs are no longer firing and current flows through the contactor. This is very efficient.

AC drives are typically 95% to 98% efficient. During start, run and stop, active components such as IGBTs are always on, causing the drive to be less efficient. However, since AC drives control both voltage and frequency, there are more opportunities for energy saving.

When operating at full speed and properly loaded, soft starters are more efficient than AC drives.

When a soft starter is up to speed and bypassed, the motor current flows across the bypass contactor. No active solid-state components are on to generate heat.

When running, an AC drive is inherently hotter than a soft starter due to the active components constantly controlling voltage and frequency.

Soft starters can yield significant cost, size and energy savings over AC drives if selected properly. AC drives can be 1.5 to 7 times the cost of a soft starter. AC drives can be 2.5 to 10 times the size of a soft starter.

Application characteristics are very important when choosing a soft starter or an AC drive. Both a soft starter and an AC drive can start a motor with reduced voltage and current, but the unique characteristics of your application will dictate which is the best choice, allowing you to right-size either a soft starter or drive.

No matter which motor control method you select, additional considerations for installation, commissioning, and additional filters or circuit protection must be considered when specifying a soft starter or AC drive.

Visit our web site to learn more about our family of low voltage soft starters and our low voltage AC drives.

Richard Anderson
Richard Anderson
Product Manager, Rockwell Automation
Richard Anderson

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